1. Field of the Invention
The present invention relates to a thin alkali metal film member and a method of producing the thin alkali metal film member, and more specifically, to a lithium metal thin film member used for a lithium cell and the like and a method of producing such a lithium metal thin film member.
2. Description of the Background Art
Recent years have seen advances of more compact and lighter electronics, so that cells as the power source for the electronics are required to be more compact and lighter. For such a requirement, a cell using lithium metal for its negative electrode can be useful and notable. In such a cell, it is also desirable to reduce the thickness of lithium metal so that the size of the cell can be reduced as much as possible.
A lithium metal foil or leaf, for example, is produced by rolling. Japanese Patent Laying-Open No. 10-58007 discloses an example of such a process, but the thickness cannot be thinner than 20 xcexcm. In addition, lithium metal is highly reactive to water and readily degrades when exposed to the air.
On the other hand, solid secondary cells using a thin lithium film have been proposed. Japanese Patent Laying-Open No. 62-44960 discloses a process of a solid secondary cell, in which a thin film of titanium disulfide as a positive electrode, a thin film of Li2Oxe2x80x94Al2O3 as an electrolyte, and a thin film of Li as a negative electrode are orderly formed on a substrate in an ionized cluster beam evaporation system. Moreover, Japanese Patent Publication No. 5-48582 discloses an electrolytic material for such a solid cell. These documents related to solid cells, however, do not suggest the techniques for independently producing a negative electrode itself with lithium.
One object of the present invention is to provide a technique for producing a thinner lithium metal film that is applicable to a cell.
Another object of the present invention is to provide a lithium metal film member that is significantly thinned, uniformly formed, and not degraded by air.
The present inventors have found that a lithium metal film of 20 xcexcm or less in thickness can be formed by a vapor deposition method, so that the present invention has been made.
According to the present invention, a thin alkali metal film member is provided, which includes a substrate and a thin film formed on the substrate by a vapor deposition method and made of a material selected from the group consisting of alkali metals and alkali metal alloys.
The thin alkali metal film member according to the present invention may have a thickness of 0.1 xcexcm to 20 xcexcm. In order to prevent the formation of pinholes, the thickness of the thin film is preferably at least 0.1 xcexcm. The thickness of the thin film is preferably in the range from 0.1 xcexcm to 20 xcexcm, and is more preferably in the range from 1 xcexcm to 10 xcexcm.
In the thin alkali metal film member according to the present invention, the thickness of the thin film may have an in-plane variation within xc2x130%. In other words, variation in the thickness of the thin film is preferably within xc2x130% of the average thickness of the thin film over the cross section of the film.
In the thin alkali metal film member according to the present invention, the substrate preferably has a thickness of 1 xcexcm to 100 xcexcm. The substrate may be made of a material selected from the group consisting of metals, alloys, metal oxides, and carbon. The substrate is preferably made of a material selected from the group consisting of copper, nickel, aluminum, iron, niobium, titanium, tungsten, magnesium, gold, silver, platinum, alloys composed of two or more metals from the foregoing, and stainless steel.
Typically, in the present invention, the thin film is made of a material selected from the group consisting of lithium and lithium alloys. In particular, the present invention is directed to an electrode member for a lithium cell.
According to the present invention, a method of producing a thin alkali metal film member is provided, which includes the step of forming a thin film made of a material selected from the group consisting of alkali metals and alkali metal alloys on a substrate by a vapor deposition method.
For example, the vapor deposition method is any one selected from the group consisting of sputtering, vacuum evaporation, laser ablation, and ion plating. The vacuum degree of the background in the vapor deposition method is preferably 1.33xc3x9710xe2x88x924 Pa (1xc3x9710xe2x88x926 Torr) or below, since oxidation of the thin alkali metal film or degradation thereof by moisture can occur in low vacuum. In the vapor deposition method, the atmosphere under which the thin film is formed is preferably constituted of a gas inactive to an alkali metal, particularly to lithium, which includes helium, neon, argon, krypton, or a mixture gas of two or more from the foregoing. The purity of the gas constituting the atmosphere is preferably at least 99.99% so that no degradation by moisture occurs in the thin lithium metal film.
The substrate on which the metal thin film such as of lithium is to be deposited may be made of a metal, an alloy, a metal oxide such as SnO2, an electrically conductive carbon such as graphite, or the like. As the metal or the alloy, any one of copper, nickel, aluminum, iron, niobium, titanium, tungsten, indium, molybdenum, magnesium, gold, silver, platinum, or an alloy of two or more metals from the foregoing, or stainless steel may be used. The substrate preferably has a thickness of at most 100 xcexcm in order to reduce the size of the lithium cell or the like. The thickness of the substrate is preferably at least 1 xcexcm in order to improve the strength of the substrate. Therefore, the thickness of the substrate may be in the range of 1 xcexcm to 100 xcexcm, and may be in the range of 1 xcexcm to 20 xcexcm for compactness.
When an alkali metal as a source material for forming a thin film is introduced into a thin film deposition system, and when the thin alkali metal film formed is taken out from the deposition system, it is undesirable to expose the source material or the thin alkali metal film to the air, since such exposure causes degradation of the source material or the film by moisture. Thus, a closed container is preferably used, so that the source material may be taken out from the container in a chamber attached to an inlet of the thin film deposition system and then the source material may be introduced into the thin film deposition system. Preferably, the formed thin alkali metal film is also transferred to a container in a chamber attached to an outlet of the thin film deposition system and then the container holding the film is closed and taken out from the system into the air.
In a preferred embodiment, the process according to the present invention further includes the steps of: taking out from a closed container a material selected from the group consisting of alkali metals and alkali metal alloys in a chamber space which is substantially inactive to the alkali metals and which is insulated from air and provided adjacent to the apparatus for forming the thin film; and transferring the material from the chamber space into the apparatus without exposing the material to the air. The material transferred into the apparatus is used to form the thin film. In a preferred embodiment, the process according to the present invention further includes the steps of: transferring a member, in which the thin film is formed, from the apparatus for forming the thin film into the chamber space which is substantially inactive to the alkali metals and which is insulated from air and provided adjacent to the apparatus for forming the thin film; and placing the transferred member into a closed container in the chamber space.
The chamber space attached to the inlet of the thin film deposition system, the chamber space attached to the outlet of the thin film deposition system, and the thin film deposition system itself may be filled with a gas of helium, nitrogen, neon, argon or krypton, or a mixture gas of two or more from the foregoing. The purity of these gases is preferably at least 99.99%. Alternatively, dry air having a dew point of xe2x88x9250xc2x0 C. or below may be used in place of such inactive gases. Preferably, the thin film deposition system and the chamber spaces adjacent thereto are filled with the above gas at the time when the source material is taken out to be transferred into the deposition system or when the thin film member prepared is transferred into the chamber space to be placed in a closed container.
The process according to the present invention is applicable to the production of an electrode member for a lithium cell, and in particular, of a negative electrode member for a lithium secondary cell. In this case, a thin film of a material selected from the group consisting of lithium and lithium alloys may be formed on the substrate by a vapor deposition method. In this case, the thin film preferably has a thickness of 20 xcexcm or less. On the produced negative electrode member for a lithium cell, a thin film of an inorganic solid electrolyte may be formed by using a suitable method such as a vapor deposition method to give a negative electrode for a lithium secondary cell.
Thus, according to the present invention, a method of producing a negative electrode member for a lithium cell is further provided. The method includes the step of forming a thin film made of an inorganic solid electrolyte on the thin film made of a material selected from the group consisting of lithium and lithium alloys in the negative electrode member for a lithium cell obtained by the above-described process. The thin film made of the inorganic solid electrolyte typically contains components A to C as follows:
A: lithium, the content of which is in the range of 30% to 65% by atomic percent;
B: one or more elements selected from the group consisting of phosphorus, silicon, boron, germanium, and gallium; and
C: sulfur.
The thin film made of the inorganic solid electrolyte may further contain at least one of oxygen and nitrogen. The content of element B is typically in the range of 0.1% to 30% by atomic percent. The content of element C is typically in the range of 20% to 60% by atomic percent. The content of one or both of oxygen and nitrogen is typically in the range of 0.1% to 10%.
In the method of producing a negative electrode member for a lithium cell according to the present invention, the ionic conductance (conductivity) at 25xc2x0 C. of the thin film made of the inorganic solid electrolyte may be at least 1xc3x9710xe2x88x924 S/cm.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.